The present invention relates generally to a method and apparatus for measuring the position of an object, and more specifically to position sensors that utilize side-emitting optical fibers.
Non-electrical position sensors are desirable for use in hazardous environments, e.g., for measuring the liquid level in gasoline or jet fuel tanks. All-optical position sensors based on the use of fiber optics would be an attractive choice because they would introduce no electrical energy, be insensitive to electromagnetic interference, have few moving parts (if any), and could provide continuous measurements.
Large-scale rotary drilling for oil and gas, minerals, and water wells, have a need for measuring the depth of drill bits and pipe segments. Measuring the length of a cable played out is often inaccurate because the cable stretches under heavy loads. An auxiliary cable under minimal tension can be used, but it can interfere with the drilling operation and is generally considered to be undesirable. Hence, a need exists for a long-range position sensor (e.g., greater than 10 meters), that is non-contact, simple, cheap, reliable, compact, non-electrical, and robust (i.e., able to withstand drilling mud and debris).
One approach is to use optical position sensors based on fluorescent fiber optics (e.g., U.S. Pat. No. 6,965,709 to Weiss). The operation of these sensors depends on the fluorescence generated in the fiber by an external pump source and its subsequent absorption as it is guided toward either end of the fiber. Since the fluorescence is generated isotropically within a small, localized region; and since its subsequent absorption depends on the path length traversed within the fiber, the ratio of optical signals outputted at the two ends can be used to determine the position of the external pump source, regardless of the individual strengths of the output signals.
However, since these types of sensors rely on fluorescence, the wavelength of the excitation (i.e. pump) light is typically limited to a fairly-narrow absorption band where stimulation (i.e., pumping/excitation) occurs. Hence, a needs exists for an optical position sensor that does not require the use of fluorescent dopants; that can operate over a wide range of wavelengths; and that has a reduced cost.
These, and other features, have been achieved in the present invention by replacing the fluorescent fibers with side-emitting fibers, which operate over a much wider range of wavelengths, and at a reduced cost.